Tin or tin alloy plating or coating which is low in cost, is resistant in corrosion and also is good in apPearance and further is excellent in solderability, is in wide use for the surface treatment or production of metal articles such as various ornaments, electrical or electronical parts typified by lead frames, printed circuit boards, contacting elements such as electrical contacts and terminals as well as structural members or parts of ferrous or ferrous alloy sheets or others.
Recently, as a method of mounting elements such as semiconductor chips, a packaging system called a film carrier (called also as a tape carrier) has been developed.
This film carrier system, as illustrated in FIG. 1, comprises a continious length of tape 2 having two rows of sproket holes along and adjacent the opposite edges thereof. The tape 2 consists of a base of a polyimide, polyester, polyeter sulfone (PES), poly-parabonic acid (PPA) or other resin and a copper foil bonded onto the base. The copper foil is formed with inner leads 3 chip bonding fingers and outer leads 4 fingers for external connections by photoeching.
In the specification, micro patterns associated with the inner leads 3 and the outer leads 4 are collectively called "lead portion".
As shown in FIG. 1, around a device hole formed by punching a central portion of the base for mounting a semiconductor chip or the like, inner leads formed from the copper foil are arranged in a high density so as to partially project into the device hole. The individual leads sometimes have a width as narrow as several ten microns.
In such fine patterns of the lead portion formed by etching the copper foil laminated on the film base as described above, tin or tin alloy plating is usually provided for the purpose of facilitating the bonding of the inner leads with a semiconductor chip bumps and of increasing the bonding strength therebetween and further for the purpose of enhancing the solderability of the outer leads.
However, tin or tin alloy plated articles have a serious problem that tin whiskers tin acicular crystals are liable to generate from the plated surfaces which can mar the appearance of ornaments and can cause shortings of electrical parts and can impair structural parts or the like.
To prevent this whisker generation, diverse methods have hitherto been proposed.
They include, for example, (1) heat treatment after tin plating as described in Japanese Patent Application Publication No. 47995/1981, (2) alloy coating, namely codeposition of tin with another metal such as lead, nickel, copper, bismuth, antimony etc. and (3) Periodic reversal of the polarity of the positive and negative electrodes involving in the plating or the use of ultrasonic energy to minimize the hydrogen occlusion in the plated metal.
These prior art approaches for preventing the generation of whiskers still have varied difficulties in practice.
For example, the method (1) requires much time for the treatment and has an upper limitation of temperature to be applicable because heating conditions sometimes can melt or soften a plastic base to deform it. Also, in the case where the heating is conducted in air, tin plated surface is oxidized, leading to decreased corrosion resistance and solderability of the surfaces of electronic parts. The method (2) has the possibilities of causing other problems or troubles due to decreased corrosion resistance and solderability or due to lowered electrical characteristics. The method (3) may be effective only when the whisker occurs by the influence of hydrogen occlusion, but is not effective when the whisker generation is caused by other factors.
Further, the applicant previously succeeded in preventing the whisker generation in connection with tin or tin alloy plated film carriers by plating an indium layer onto a tin or tin alloy plated layer, and have obtained good fruits as described in U.S. Ser. No. 282,183 filed on Dec. 9, 1988 or European Patent Application No. 88120838 Filed on Dec. 13, 1988.
However, there are many applications which require by all means that exposed surface layer is a tin or tin alloy layer.